hmmm honestly not sure Sam , I'd be pretty confident you could just get it here , I know , if using an agent anything can happen 5555 ..And there is a bit more to the marriage Visa , rather than retirement.
Yes , not uncommon , but just apply again and keep applying , add some more info as to why she MUST return , give them more than they request , you are a great chance of success second time around .
I think 20k is fine , it will be a one off , if you were using an agent it would be around 16k , and this is a qualified individual, it matters not the price in your home country , not even remotely relevant .
Just pay the money , get the Visa , consider yourself lucky , it's cheaper than leaving , and Sabai Sabai .
The only other alternative is to find a different country , but on your writings you will is to stay here - Just pay ! It's a cost of your stay , a Visa cost .
So ( as an absolute bare minimum) after transport and accomodation ( that's AFTER ) you need at least 10k baht per week x 4 = 40 k say plus 15k plus 10 = 65 k for an individual , now this is also the amount needed for you Visa , for 2 people , times it by 2 .
First time , I paid 33k for 15 months , but that was with multiple exit which was 4.3k from memory ..
A very reasonable agent , therefore for 15 months , bank account set , etc etc so that's around 29k with no exit , and that's a good deal , certainly in our local office .